The present invention relates to a highly reliable online system for a computer system having a database.
Conventionally, the following methods have been taken for improving the reliability of online systems. FIG. 38 is a diagram for explaining such methods.
A computer center 11 is provided with duplicate external storage units 13, 14 for storing database records 15, 16 and updating the same in an on-line fashion. The database records 15, 16 are updated, when necessary, simultaneously from a central processing unit 12 (1). Thus, even if one of the external storage units 13 or 14 is unusable due to a failure, a duplicate storage of the identical database in different external storage units permits utilizing the database records stored in the other external storage unit.
Also, a backup of a database (DB) is stored in an external storage unit 17 which is not connected to the online system (2), and the storage unit 17 is preserved in the same computer center 11 to which the online system is connected or in another warehouse or a computer center 18 so as to allow the database to be recovered in case the external storage units 13, 14, or even with the external storage unit 17 included, are simultaneously unusable.
This type of art is disclosed, for example, in Japanese Patent Laid-open JP-A-61-196347 (1986) and so on.
The above-mentioned prior art implies the following problems in the online system which requires a high reliability, as will be hereblow explained with reference to FIG. 39:
(1) If the computer center 11 suffers from a disaster (A), terminals 24a, 24b, 24c, 24d become all inoperable. The recovery of the database is also impossible due to such a disaster.
(2) If a wide area 21 including the computer center 11 (an area including a plurality of adjacent prefectures, cities, towns or villages) suffers from a disaster (B), not only the terminals 24a, 24b included in the computer center 11 but also the terminals 24c, 24d included in a wide area 25, which was not hit by the disaster, becomes unusable for online operations. The recovery of the database is neither possible due to this type of disaster.
(3) If a disaster occurs in a transmission line 26b connecting a computer 23 and the terminals 24c, 24d for operating the computer between the wide area 21 including the computer center 11 and another wide area 25 (C), the terminals 24a, 24b included in the wide area 21 can be used for online operations, whereas the terminals 24c, 24d in the wide area 25 cannot be used.
(4) When the computer center needs a construction work on a large scale for maintaining the reliability of hardware, attending to the operating performance or the like, it is necessary to temporarily interrupt the online service.